1. Field of the Invention
This invention relates generally to projectile guidance systems, and more specifically to an apparatus that provides a local vertical reference for a remotely-guided projectile.
2. Description of the Prior Art
State of the art projectile guidance systems include one or more projectiles and one or more launching devices. Many present-day launching devices commonly include a guidance computer that issues control signals to the projectile. A plurality of control signals are used, and each signal corresponds to a particular projectile steering command. The projectile contains steering control hardware responsive to these control signals that steers the missile on a line-of-sight trajectory toward a desired target.
To generate synchronous steering commands for guiding a rotating projectile along a desired trajectory, the local vertical reference of the projectile is required. At least part of the vertical reference determination mechanism is usually mounted in the projectile or missile. Therefore, such mechanism should include hardware having relatively small size to minimally affect the aerodynamic properties of the missile. To minimize the amount of onboard hardware, it is desirable to situate at the launch computer as many components as feasible for the determination of local vertical reference.
Many prior art guidance systems derive vertical reference information from on-board gyroscopes. Although these systems are satisfactory for some applications, gyroscopes are not practical for small missiles of less than about 60 millimeters in diameter. Furthermore, gyroscopes are ineffective when employed in the context of roll-stabilized or other rolling or spinning missiles. Because of relatively high cost and complexity of gyroscopes, it is desirable to provide an improved alternative technique for the derivation of local vertical reference information.
Special considerations apply to the derivation of vertical reference information for high-velocity projectiles flying at about 3.5 km/sec or higher. One such consideration relates to projectile cooling. In general, the front surfaces of the projectile must be cooled to prevent erosion of the nose tip. Nose tip cooling is provided by using fluid or gas injection, or by transpiration into the airstream.
A second consideration applicable to high velocity projectiles concerns roll or spin. It is desirable to introduce a slow roll about the central axis of a high velocity projectile, not to stabilize the projectile but to cancel the net effect of aerodynamic forces arising from small asymmetries of the projectile nose and body. The necessary roll is from one to ten revolutions per second. Although various prior art projectiles require higher spin rates for stability, most present-day projectiles are sufficiently stable without such roll or spin.
A useful technique for controlling the trajectory of high-velocity projectiles is denominated "shock interaction steering". This technique operates by transmitting an injection control signal to the projectile. The injection control signal causes a fluid or gas to be injected into the airstream from the projectile at right angles to the airstream. The release of fluid or gas sets up a shock wave that exerts a force on the projectile. Depending upon the magnitude of this force and its distance from the projectile center-of-gravity, the force can rotate or translate the missile, or both. The injection control signal is synchronized with roll position by a remote steering command so that the resulting aerodynamic forces cause the desired motion. Synchronization requires the local vertical reference information to be available at the launch computer for use in preparing the steering command signals. In this manner, the injection control signals facilitate the achievement of a desired missile trajectory.
State-of-the-art methods for providing communications between the launcher and the missile involve the use of electromagnetic radiation. The launcher contains a transmitter for producing a signal composed of electromagnetic radiation having a predetermined wavelength, and a receiver responsive to this signal Electromagnetic radiation may include, for example, a continuous or a pulsed light wave. During system operation, the launcher transmit a signal toward the missile. The missile contains a reflector that returns a portion of the incident electromagnetic radiation to the launcher receiver.
FIG. 2 illustrates a side view of basic prior art retroreflector 138 that may be employed in the context of a missile guidance system. Optical source 208 at the launching site transmits a beam of optical energy through polarized mirror 207 toward the retroreflector. Optical source 208 is positioned relative to polarized mirror 207 so that mirror 207 is virtually transparent to the energy emitted by optical source 208. The optical energy travels through space and eventually intercepts first surface 143 of retroreflector 138. The optical energy incident upon first surface 143 is reflected to second surface 148 as shown.
The angle between the first surface and the second surface is approximately 90 degrees, such that second surface 148 reflects the incident optical energy back toward polarized mirror 207. During each of these reflections, the polarization of the optical energy changes. When the twice-reflected optical energy reaches polarized mirror 207, the energy is of such polarization that it is mostly reflected from polarized mirror 207 to optical detector 209. Optical detector 209 produces an output that is dependent upon the presence or absence of incident optical energy.
One example of a prior art projectile reflector is disclosed in U.S. Pat. No. 4,990,918, issued to Michelson et al. The reflector consists of a trihedral corner reflector arrangement having three planar faces at right angles to one another. The first planar face has a triangular shape, and contains a right angle that forms a common vertex. The second and third planar faces are joined along an inner edge to form a center line extending from the common vertex. The reflector is symmetrical about the center line.
The purpose of the Michelson reflector is to enhance the radar cross-section of fixed and moving targets. The cross-sectional pattern is enhanced in one plane, thereby improving radar detectability in that plane. However, the device does not provide local vertical reference information, nor does the device provide a signal return that is strongly dependent upon the roll of the missile.
Another prior art reflector is disclosed in U.S. Pat. No. 4,709,580, issued to Butts, Jr., et al. The Butts reflector includes two intersecting reflecting planes oriented at right angles with respect to one another, such that incident laser beams that are perpendicular to the line of intersection of the two planes are reflected directly back to the laser source. Each reflector therefore has a single retroreflecting plane perpendicular to the line of intersection. The retroreflector is mounted on a spinning object. Using a three-layered incident beam and a three-layered receiver, the rate of spin of the object may then be mathematically determined. Accordingly, the Butts reflector is designed for the purpose of ascertaining rate of spin. Butts does not disclose a technique for determining the local vertical reference of a moving object.
U.S. Pat. No. 4,047,816, issued to Pell et al., describes a technique for determining the attitude of a flight vehicle. Two ground-based laser transmitter/receiver stations are employed. A skewed reflector is mounted on the flight vehicle. As the vehicle rotates, each plane of retroreflected energy sweeps each of the ground stations at a time interval dependent upon the attitude of the vehicle. The reflector alignment on the surface of the vehicle is known, and the roll rate is measured by a signal reflected from the skewed retroreflector. This skewed reflector method of measurement is disadvantageous in the context of missile launching systems because two ground-based tracking stations are required to ascertain the roll rate of the vehicle/projectile.
A retroreflector development for use in the missile launching system environment is described in Miller, Jr., et al., U.S. Pat. No. 4,072,281. The Miller device employs a ground-based laser beam transmitter having a predetermined beam polarization. The missile contains a light detector that detects polarized light filtered through a prism. The output of the detector is processed by electronic circuitry mounted onboard the missile to determine the roll angle of the missile relative to the laser beam polarization axis at the launch site. In this manner, the vertical reference axis of a rolling missile may be obtained.
The Miller system does not disclose the determination of the local vertical reference of the missile through the use of a retroreflector having a canted surface or a single facet. Rather, the Miller system uses a prism responsive to incoming light waves of a predetermined polarization and requires the mounting of electronic circuitry within the missile. Thus, Miller discloses a missile with active onboard apparatus for determining the local vertical axis. This circuitry consumes valuable space within the projectile and increases the cost and complexity of each missile.
It is often desired to launch a plurality of missiles from a single launcher. Accordingly, it would be advantageous to locate hardware at the launcher whenever possible. The hardware mounted onboard the missile should be fully passive, such that all active, power-consuming electronic circuitry is contained at the launching device. A system such as Miller's that requires each missile to be equipped with hardware does not represent an economically efficient system. In general, it is much more economically efficient to build hardware once, for permanent use at the launcher, instead of constructing a plurality of expendable hardware devices for single use aboard a missile. Furthermore, the use of hardware mounted within the missile adds undesirable weight to the missile.
What is needed is a system that is capable of remotely determining the local vertical reference of a line-of-sight projectile. The system should require a minimum amount of hardware at the missile. The system should be equipped to determine The local vertical reference of a line-of-sight projectile from a single missile launcher site. Additionally, the system should be sensitive to the roll or attitude, or both, of the missile.